Poor Sheldon. One week, he’s tossing off his first “I love you” to his girlfriend; the next week he’s freaking out about his best friend Leonard dying during a routine surgery. It’s getting so a socially inept guy isn’t allowed to keep a single emotion suppressed around here.

But it was a sweet and appropriately Sheldon response from Mr. Cooper to act in an irrational way to Leonard’s surgery to repair a deviated septum. Not only because it afforded the show an opportunity to focus on the relationship that started The Big Bang Theory, but also because, after such a big-boy development with Amy last week, it makes sense that emotionally immature Sheldon might not be ready to deal with the thought of a Leonard-less world.

And, most important, it brought about some funny Sheldon missives and the chance for Jim Parsons to show off his physical comedy chops, of which he has an impressive reserve.

Sheldon pretends, initially, to be upset about the septum surgery because it will put the kibosh on Leonard’s snoring, a nuisance Sheldon has come to appreciate as a “mucus-powered white-noise machine.” He begins researching how the surgery could go wrong — the odds of death during the procedure is 1 in 700,000 — but Sheldon concocts sundry ways around that number (sepsis, latex allergy, asteroid strike) and revises the odds of death to a “sphincter-tightening” 1 in 300.

Leonard promises to reconsider, but when an early appointment opens up, he takes it, and tells Sheldon he’s going swimming in a public pool. While Amy’s driving Sheldon to work, Sheldon realizes Leonard was lying (because he didn’t take his public-swimming shirt with him) and demands Amy take him to the hospital to see his pal.

An earthquake tremor knocks the lights out at the hospital, sending Sheldon on a panicked trip to check on Leonard in the operating room. The only thing standing between him and his buddy? A glass door, which he doesn’t see — the lights — and runs right into, nose first. You see where this is going, right? Leonard and Sheldon, sitting on the couch in their apartment, each with a bandaged nose and a nasally voice. “You looooove me,” Leonard says, while Sheldon warns him not to be smug: An inattentive surgeon could have allowed a spider to lay eggs in his nose.

In the crisscrossing subplots, Raj is trying to come up with a 40th-anniversary present for his parents when he gets a jolt: no gift necessary, because Mom and Dad are breaking up. Howard and Bernadette try to comfort him, but news that the Koothrappali marriage is ending because of years of built-up resentments sends the Wolowitzes scrambling to avoid the same fate. They decide to unfurl a list of all the things they love about each other, but that turns into a conversation in which they snipe at each other under the guise of compliments. (“I love that you have the confidence to speak … even without giving it an ounce of thought.”)

It’s a setup of story lines we’re almost certainly going to revisit this season. Raj is in a new relationship, and this new development with his parents’ marriage could mess with how he proceeds with Emily. The episode also briefly, but interestingly, touches on how, no matter your age, finding out your 'rents are splitting up is painful.

As for Howie and Bernie, it’s another nice touch to have them realize, preemptively, that their relationship could use some tending to. Next week’s episode involves a story line in which Bernadette is forced to face up to the fact that many people see her as a pint-size but still wickedly intimidating bully, so maybe that bit of forced introspection will have some add-on benefits for the Wolowitz marriage.

Now the saddest Big Bang Theory news: Chuck Lorre’s vanity card was replaced by a remembrance of Carol Ann Susi, the actress who was loudly heard but never seen as Howard’s mom, Debbie Wolowitz. “In loving memory of Carol Ann Susi, Mrs. Wolowitz. Every time you spoke, we laughed. You’re in our hearts forever,” a tribute read under a photo of the actress, who died on November 11 after a brief battle with cancer.

Susi had a long TV résumé — including that memorable guest-appearance on Seinfeld as Carrie, the woman who was unimpressed with George’s Big Mac date and hardware-store job aspirations — but her work as Howard’s mom is her signature role, and her “HOW-AHD!”s will be greatly missed. It’s extra sad that the character was in the middle of her best story line ever, as Debbie’s friendship — or whatever it is — with comic-book-store guy Stuart has been such a source of irritation to Howard and amusement to viewers. Aside from Shamy’s future and the will-they-won’t-they-get-married story with Penny and Leonard, the Deb/Stuart relationship is the plot I’ve been most excited about this season. It was such an unexpected delight, and now we’re left to wonder how the series will deal with Susi’s death. We all need a hot beverage.

THEOR-EMS:

• Howard to Raj: “I found this fencing school in Burbank that has a Jedi class. If you’re cool being the only adults there, they said they are.”

• Sheldon, on seeing Jay Z’s name: “I’m not sure if that’s a person or a typo.”

• Amy, unlike Leonard, allows Sheldon to eat sticky fast-food French-toast sticks in her car. Doesn’t that seem like a food Sheldon would consider too messy to eat, especially in a moving vehicle?

• Leonard: “I’m not going to die.” Sheldon: “You don’t know that.” Leonard: “I do know that it won’t be from an asteroid strike.” Sheldon: “You know who else said that? Every cocky T. rex currently swimming around in the gas tank of your car.”

• Amy: “It’s sweet that you care about him so much.” Sheldon: “I do. And I wouldn’t be able to forgive myself if something happened to him, and I wasn’t at his bedside to say, ‘I told you so.’”

• No need for Sheldon to ever waste money on expensive cologne. Amy reveals the scent that turns her on: “I was distracted. He has on extra baby powder today.”

• Bernadette learns Howard once went to couples therapy. She assumes it was with his mom. It was with Raj.

• The inscription on the urn Sheldon has engraved for Leonard: “Here lie the ashes of Leonard Hofstadter/He thought he was right, but his roommate knew better.” On the matching urn Sheldon had engraved for himself: “I’m with stupid.”

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